System for controlling flue gas exit temperature for optimal scr operations

ABSTRACT

A system for maintaining an optimal flue gas inlet to a boiler mounted SCR assembly in the flue of the boiler is accomplished by mixing the normal inlet feedwater to an economizer of the boiler with near saturation water from downcomers of the boiler to thereby raise the temperature of the flue gas passing across the economizer and raising the SCR inlet to the desired optimal SCR operation temperature.

FIELD AND BACKGROUND OF THE INVENTION

[0001] The present invention is generally drawn to boilers using SCR(Selective Catalyst Reduction) systems at the flue exhaust to clean theexhaust gas thereby and more particularly to the optimized temperatureoperation of same.

[0002] In operating a boiler with a Selective Catalytic Reductionsystem, or SCR, at the flue gas exhaust, the reactiveness of thecatalyst is dependent upon the flue gas temperature entering thecatalyst reactor. A given catalyst will have maximum performance when itis operated at the temperature of peak performance (TPP). As an example,in a typical SCR for NO_(X) removal, the temperature of peak performance(typically 650° F.) at the reaction of ammonia with NO_(X) present inthe flue gas is optimized and the amount of the ammonia needed for thecatalytic reaction is minimized. Therefore, for economic reasons thedesired gas temperature entering the catalyst reactor should bemaintained at the TPP at all loads. Also, maintaining the desired fluegas temperature reduces the formation of ammonia and/or sulfate saltswithin the ammonia injection grid (AIG) and the catalyst.

[0003] However, as boiler load decreases, the boiler exit gastemperature will drop below the TPP. To increase the gas temperature toTPP, current practice has been to use an economizer gas bypass. Theeconomizer gas bypass is used to bypass the hotter gases upstream of theeconomizer to the cooler gas that leaves the economizer and mixes withthe flue gas. By controlling the amount of gas that passes through thebypass system, a boiler exit flue gas temperature of approaching the TPPcan be maintained at the lower boiler loads which normally results inthe flue gas temperature below TPP.

[0004] Also, systems for mixing economizer feedwater with hot water atthe inlet of the economizer are known. These systems were known as theOff Line Circulation System and were developed in the mid 1980s.However, this system was not designed for increasing the flue gastemperature from the economizer. This system's main purpose was toreduce the economizer inlet headers thermal shock that occurs duringboiler start up and shut down and to eliminate thestratification/subcooling temperature effects that occur in the furnacewalls of the boiler when the boiler is off line and put into hotstandby.

[0005] Thus, what was needed was a simpler system that required lessphysical space to obtain the desired flue gas temperature to the SCR atvarious boiler loads. With the known flue gas bypass systems currentlyused for SCR application, static mixing devices, pressure reducingvanes/plates and thermal mixing devices were required to make thedifferent temperature flue gases mix before the gas mixture reaches theinlet of the catalyst reactor. In most applications, obtaining thestrict mixing requirements for flow, temperature and the mixing of thereagent (if received) before the catalyst reactor was often difficult.

SUMMARY OF THE INVENTION

[0006] The present invention solves the problems associated with priorart devices as well as others by providing a boiler water recirculationsystem where the variation in the gas flow and temperature at theeconomizer outlet is less severe than with a flue gas bypass system,making it easier to meet the gas mixing requirement for the catalystreactor at the optimal inlet temperature.

[0007] To accomplish this, the invention uses the economizer to increasethe outlet temperature of the flue gases to the desired temperature atthe lower boiler loads by using a boiler recirculation system to providehigher temperature water from the circulation system that is used tocool the furnace walls. The recirculation system supplies nearsaturation water from the downcomers of drum circulation boilerapplications, or for once-through boiler applications, the fluid isobtained from a fluid mix location in the upper region of the lowerfurnace. In either a drum or once through boiler application, the highertemperature water is transferred to the economizer inlet and mixed withthe boiler's economizer normal feedwater inlet flow. The mixture of thetwo fluid streams results in a higher temperature fluid in theeconomizer that can be used to increase the flue gas temperature leavingthe economizer. With proper adjustment of the different fluid streams tothe economizer, the desired flue gas temperature can be obtained for anyboiler load. The amount of near saturation water (or higher temperaturefurnace wall water for a once-through boiler) from the boilerrecirculation system is controlled throughout the load range.Calculations have shown that no catastrophic effects (critical heat fluxor tube failures) on the cooling of the boiler's furnace walls willoccur in the use of this system.

[0008] In view of the foregoing it is seen that one aspect of thepresent invention is to provide stable flue gas temperature controlsystem based on economizer water inlet temperature.

[0009] Yet another aspect of the present invention is to provide anincreased temperature economizer gas outlet responsive to increasedeconomizer water inlet temperature.

[0010] These and other aspects of the present invention will be morefully understood upon a review of the following description of thepreferred embodiment when considered in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] In the drawings:

[0012]FIG. 1 is a schematic of a boiler water/steam recirculation systemutilizing the increased temperature economizer water inlet of thepresent invention.

[0013]FIG. 2 is a schematic of the control system used to increase fluegas temperature in response to increased economizer water inlettemperature.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Referring to FIGS. 1 and 2 of the drawings, the present inventionuses a different approach to obtaining a TPP boiler exit flue gastemperature. In a normal boiler application, the water side of theeconomizer is used to cool the flue gas that flows over the surface thatis installed in the boiler. Here, the boiler recirculation system (10)is modified to have higher temperature water near saturation fromdowncomers (12) connected by a bypass line (14) to an inlet (16) of aneconomizer (18). The inlet (16) is a tee inlet with the other inlet ofthe tee providing normal feedwater flow from line (20). The flow throughline (14) is provided by a pump (22) which has monitoring, flow F andpressure P sensors mounted on both sides of the pump (22). An economizer(18) bypass line (24) is provided as shown in dotted lines on FIG. 1, torecirculate the downcomers (12) saturated water back thereto from drum(26) when no increased water temperature is needed for mixing with thenormal economizer (18) feedwater from line (20).

[0015] With particular reference to FIG. 2, it will be seen that theoperation of this invention is as follows. An SCR (28) located on anoutlet (30) of a boiler flue (32) needs the optimum flue gas temperaturesupplied to the inlet thereof for optimal operation as was describedearlier. To accomplish this end, a temperature sensor (34) is mounted ina flue (32) near the entrance to the SCR (28) ton monitor the flue gastemperature. A signal indicative of the actual flue gas is transmittedalong line (36) to comparator station (38) having a set point signal ofthe optimum temperature inputting thereto along line (40). Anydifference in these two signals develops an error signal e along line(42) to a controller (44) which controls the opening of a gate valve(46) to control the quantity of saturation temperature water sent alonglone (14) to the tee (16) to be mixed with the normal temperaturefeedwater from line (20) and supplied to the economizer (18).

[0016] The bypass line (24) is closed by normally closed valve (48)being maintained closed by the error signal e being transmitted alongline (50) to a NAND gate (52). As long as there is a positive signalfrom comparator (38) to the NAND gate (52), there will be no controlsignal passed therefrom along line (54) to the valve (48) and it willremain shut. When the error e signal becomes O indicating a flue gastemperature is at the optimum, a O signal will enter NAND gate (52)along line (50) and a O signal will enter the NAND gate (52) along line(56)from the controller (44). This will cause an output control signalto be transmitted along line (54) to normally closed valve (48) to openand a control signal along line (58) to the normally open valve (46) toclose. This establishes flow back to the downcomers (12) bypassing theeconomizer (18) until the flue (32) temperature falls below 650° F. andsaturated water will again be mixed with normal feedwater to theeconomizer (18) inlet.

[0017] Clearly as more saturated water in inputted to the inlet of theeconomizer (18) the flue temperature across the economizer (18) willrise and, when mixed with normal flue gas, will raise the temperature tothe temperature of peak performance at the SCR (28) inlet.

[0018] Certain modifications and construction details have been deletedherein since they are obvious to those of ordinary skill in the art areaand for the sake of conciseness and readability but are properly withinthe scope of the following claim.

We claim:
 1. A system for maintaining an optimal flue gas temperature tothe inlet of an SCR assembly mounted therein comprising; a boiler havingan economizer mounted in the flue thereof; a boiler downcomer havingwater therein near saturation temperature; an economizer water inletproviding a mixture of normal feedwater and water from said downcomer;and a control system for mixing the water inlet to insure that the fluetemperature of the inlet to the SCR is optional.
 2. A system as setforth in claim 1 wherein said control system includes a temperaturesensor mounted at the inlet of the SCR to monitor flue temperature and acontroller to vary the quantity of water from said downcomer to saideconomizer water inlet in response to a difference between optimal SCRinlet temperature and actual inlet temperature.
 3. A system as set forthin claim 3 including a controller connected to said comparator forcontrolling a valve varying the water flow from said downcomer to saideconomizer inlet.
 4. A system as set forth in claim 3 including acontroller connected to said comparator for controlling a valve varyingthe water flow from said downcomer to said economizer inlet.
 5. A systemas set forth in claim 4 including a bypass for connecting the water flowfrom said downcomer to the water outlet of said economizer when the SCRinlet temperature was optimal.
 6. A boiler water recirculation systemcomprising; a boiler having downcomers and an economizer connected to aboiler drum; and said economizer having an inlet for mixing normalfeedwater with water from said downcomers.
 7. A boiler recirculationsystem as set forth in claim 6 wherein the mixing of water from saiddowncomer in said inlet is done with a fixed feedwater flow.
 8. A boilerrecirculation system as set forth in claim 6 wherein the amount of waterfrom said downcomers is proportional to the difference between actualand desired flue gas temperature from the boiler.